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Flow Reduction (flow + reduction)
Selected AbstractsIs structure or function a better measure of the effects of water abstraction on ecosystem integrity?FRESHWATER BIOLOGY, Issue 10 2009RUSSELL G. DEATH Summary 1. Assessments of flow abstractions in streams often focus on changes to biological communities and in-stream physical characteristics, with little consideration for changes in ecosystem functioning. It is unclear whether functional indicators of ecosystem health may be useful for assessing the impacts of reduced discharge on small streams. 2. We used weirs and diversions to reduce stream discharge by over 89% in three small New Zealand streams (11,84 L s,1), ranging in water quality from pristine to moderately impaired. 3. We used both structural (benthic invertebrates) and functional (drifting invertebrates, leaf breakdown, coarse particulate organic matter (CPOM) retention and primary productivity) measures of ecosystem integrity to compare responses to water abstraction in before-after, control-impact designed experiments during summer 2005. 4. At the pristine site, the density of invertebrates, taxon richness, Macroinvertebrate Community Index (MCI), Quantitative MCI, percentage of Ephemeroptera, Plecoptera and Trichoptera individuals and percentage of filter-feeders decreased in response to reduced flows. Only taxon richness decreased at the mildly impaired stream, and reduced discharge had no effect on the invertebrate community at the stream with the lowest water quality. 5. We found that reduced discharge had little influence on the breakdown rate of willow leaves in mesh bags over 1 month. Primary productivity was also relatively insensitive to water abstraction. However, CPOM retention increased with decreased flows. Drift propensity of invertebrates increased at two sites but only within the first few days after flow reduction. 6. Structural measures of ecosystem integrity suggested that the impacts of water abstraction differed among streams of varying water quality, probably because of differences in the sensitivity of invertebrate assemblages in the three streams. In contrast, the three functional measures tested were generally less sensitive to water abstraction impacts, although understanding how stream ecosystems respond to water abstraction clearly requires that both are considered. [source] Ecological effects of perturbation by drought in flowing watersFRESHWATER BIOLOGY, Issue 7 2003P. S. Lake SUMMARY 1Knowledge of the ecology of droughts in flowing waters is scattered and fragmentary, with much of the available information being gathered opportunistically. Studies on intermittent and arid-zone streams have provided most of the information. 2Drought in streams may be viewed as a disturbance in which water inflow, river flow and water availability fall to extremely low levels for extended periods of time. As an ecological perturbation, there is the disturbance of drought and the responses of the biota to the drought. 3Droughts can either be periodic, seasonal or supra-seasonal events. The types of disturbance for seasonal droughts are presses and for supra-seasonal droughts, ramps. 4In droughts, hydrological connectivity is disrupted. Such disruption range from flow reduction to complete loss of surface water and connectivity. The longitudinal patterns along streams as to where flow ceases and drying up occurs differs between streams. Three patterns are outlined: ,downstream drying', ,headwater drying' and ,mid-reach drying'. 5There are both direct and indirect effects of drought on stream ecosystems. Marked direct effects include loss of water, loss of habitat for aquatic organisms and loss of stream connectivity. Indirect effects include the deterioration of water quality, alteration of food resources, and changes in the strength and structure of interspecific interactions. 6Droughts have marked effects on the densities and size- or age-structure of populations, on community composition and diversity, and on ecosystem processes. 7Organisms can resist the effects of drought by the use of refugia. Survival in refugia may strongly influence the capacity of the biota to recover from droughts once they break. 8Recovery by biota varies markedly between seasonal and supra-seasonal droughts. Faunal recovery from seasonal droughts follows predictable sequences, whilst recovery from supra-seasonal droughts varies from one case to another and may be marked by dense populations of transient species and the depletion of biota that normally occur in the streams. 9The restoration of streams must include the provision of drought refugia and the inclusion of drought in the long-term flow regime. [source] Semianalytical Solutions for Stream Depletion in Partially Penetrating StreamsGROUND WATER, Issue 1 2004Xunhong Chen In the analysis of streamflow depletion, the Hunt (1999) solution has an important advantage because it considers a partially penetrating stream. By extending the Hunt drawdown solution, this paper presents semianalytical solutions for gaining streams that evaluate the induced stream infiltration and base flow reduction separately. Simulation results show that for a given ,h (the initial hydraulic head difference between stream and aquifer beneath the channel), the base flow reduction is in direct proportion to the product of streambed leakage (,) and the distance between pumping well and stream (L), and the induced stream infiltration is in inverse proportion to ,L. ,h has a significant effect on the ratio of stream infiltration to base flow reduction. The results from the semianalytical solutions agree well with those from MODFLOW simulations. The semianalytical solutions are useful in the verification of numerical simulations and in the analysis of stream-aquifer interactions where water quantity or quality is concerned. [source] Time evolution of cerebral perfusion and apparent diffusion coefficient measured by magnetic resonance imaging in a porcine stroke modelJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2002Lisbeth Røhl MD Abstract Purpose To demonstrate the feasibility of sequential diffusion-weighted (DW) and perfusion-weighted (PW) magnetic resonance imaging (MRI) of a recently developed porcine stroke model and to evaluate the evolution of cerebral perfusion and the apparent diffusion coefficient (ADC) over time. Materials and Methods In five pigs, DW imaging (DWI) and PW imaging (PWI) was carried out for 7 hours after stroke onset, starting 1 hour after middle cerebral artery occlusion (MCAO). Results The DWI lesion volume increased significantly with time, and final DWI lesion volume correlated well with lesion area on histological sections (r = 0.910). T2 changes could be recognized 3 hours after stroke onset. At 1 hour the ADC ratio (ischemic lesion/contralateral side) was reduced to 0.81 in the caudate-putamen and to 0.87 in the cortex, and the cerebral blood flow ratio was reduced to 0.40 in the caudate-putamen and 0.51 in the cortex. Conclusion The level of flow reduction in the caudate-putamen and the cortex after 1 hour is in good correlation with human thresholds of irreversible and reversible ischemic damage, and accordingly, this model might be a model for mechanisms of infarct evolution and therapeutic intervention. J. Magn. Reson. Imaging 2002;15:123,129. © 2002 Wiley-Liss, Inc. [source] Recombinant activated factor VII efficacy and safety in a model of bleeding and thrombosis in hypothermic rabbits: a blind studyJOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 2 2007A. GODIER Summary.,Background:,Recombinant activated factor VII (rFVIIa) is increasingly used to secure hemostasis in hemorrhagic situations in trauma and surgical patients. Hypothermia is often observed in these clinical settings. Objective:,To study the efficacy and safety of rFVIIa in hypothermia in a rabbit model of bleeding and thrombosis. Methods:,Sixty-nine rabbits were anesthetized, ventilated and monitored for blood pressure, temperature and carotid flow. The Folts model was used: a stenosis (75%) and an injury were carried out on the carotid artery, inducing thrombosis. Blood flow decreased as thrombus size increased until the pressure gradient was such that the thrombus was released and local arterial blood flow was suddenly restored. This is known as a cyclic flow reduction (CFR). After counting baseline CFRs during a 20-min period (P1), rabbits were randomized blindly to one of four groups: normothermic (NT) placebo or rFVIIa (150 ,g kg,1), hypothermic (HT) (34 °C) placebo or rFVIIa. Then CFRs were recorded over a second period (P2). At the end of the experiment, a hepato-splenic section was performed and the amount of blood loss was recorded. After each period, the following were measured: ear immersion bleeding time (BT), hemoglobin, platelet count, prothrombin time (PT), activated partial thromboplastin time (aPTT) and fibrinogen. Results:,Hypothermia increased BT and blood loss. These effects were reversed by rFVIIa. In NT rabbits, rFVIIa shortened BT but did not reduce blood loss. rFVIIa-treated rabbits bled similarly regardless of temperature. The incidence of CFRs was higher in treated than placebo animals regardless of temperature. rFVIIa decreased PT and aPTT without modifying platelet count or fibrinogen level. Conclusion:,Hemostatic efficacy of rFVIIa was maintained in hypothermia. However, the number of CFRs was higher in the rFVIIa-treated group than in the placebo groups, whether for NT or HT rabbits. [source] Microemboli may link spreading depression, migraine aura, and patent foramen ovaleANNALS OF NEUROLOGY, Issue 2 2010Ala Nozari MD Objective Patent foramen ovale and pulmonary arteriovenous shunts are associated with serious complications such as cerebral emboli, stroke, and migraine with aura. The pathophysiological mechanisms that link these conditions are unknown. We aimed to establish a mechanism linking microembolization to migraine aura in an experimental animal model. Methods We introduced particulate or air microemboli into the carotid circulation in mice to determine whether transient microvascular occlusion, insufficient to cause infarcts, triggered cortical spreading depression (CSD), a propagating slow depolarization that underlies migraine aura. Results Air microemboli reliably triggered CSD without causing infarction. Polystyrene microspheres (10,m) or cholesterol crystals (<70,m) triggered CSD in 16 of 28 mice, with 60% of the mice (40% of those with CSD) showing no infarcts or inflammation on detailed histological analysis of serial brain sections. No evidence of injury was detected on magnetic resonance imaging examination (9.4T; T2 weighted) in 14 of 15 selected animals. The occurrence of CSD appeared to be related to the magnitude and duration of flow reduction, with a triggering mechanism that depended on decreased brain perfusion but not sustained tissue damage. Interpretation In a mouse model, microemboli triggered CSD, often without causing microinfarction. Paradoxical embolization then may link cardiac and extracardiac right-to-left shunts to migraine aura. If translatable to humans, a subset of migraine auras may belong to a spectrum of hypoperfusion disorders along with transient ischemic attacks and silent infarcts. ANN NEUROL 2010;67:221,229 [source] Springs in time: fish fauna and habitat changes in springs over a 20-year intervalAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 6 2008Elizabeth A. Bergey Abstract 1.Despite the range of threats to springs and the number of spring-endemic species, studies of temporal changes in the fauna of springs have rarely been reported. Changes in the fish of 22 Oklahoma (USA) springs were compared among surveys in 1981, 1982, and 2001. 2.Twenty-year assemblage differences were correlated with physical alteration of specific springs and stocking of native fish, which was made possible by past habitat changes that produced pools. Physical alteration of springs is a major ongoing threat to Oklahoma springs. 3.Variation in spring fish assemblages among the three surveys was apparently affected by fish movement in and out of springs, and the greater rain-induced connectivity between springs and streams during one year. 4.Although flow reduction is a commonly cited threat to springs, there was little evidence of flow reduction impacts in this study because Oklahoma springs may have been affected prior to 1981 and high-flow springs, which most often contain fish, were in areas with low groundwater water use. Copyright © 2008 John Wiley & Sons, Ltd. [source] Microvascular Rheology and HemodynamicsMICROCIRCULATION, Issue 1 2005HERBERT H. LIPOWSKY ABSTRACT The goal of elucidating the biophysical and physiological basis of pressure,flow relations in the microcirculation has been a recurring theme since the first observations of capillary blood flow in living tissues. At the birth of the Microcirculatory Society, seminal observations on the heterogeneous distribution of blood cells in the microvasculature and the rheological properties of blood in small bore tubes raised many questions on the viscous properties of blood flow in the microcirculation that captured the attention of the Society's membership. It is now recognized that blood viscosity in small bore tubes may fall dramatically as shear rates are increased, and increase dramatically with elevations in hematocrit. These relationships are strongly affected by blood cell deformability and concentration, red cell aggregation, and white cell interactions with the red cells and endothelium. Increasing strength of red cell aggregation may result in sequestration of clumps of red cells with either reductions or increases in microvascular hematocrit dependent upon network topography. During red cell aggregation, resistance to flow may thus decrease with hematocrit reduction or increase due to redistribution of red cells. Blood cell adhesion to the microvessel wall may initiate flow reductions, as, for example, in the case of red cell adhesion to the endothelium in sickle cell disease, or leukocyte adhesion in inflammation. The endothelial glycocalyx has been shown to result from a balance of the biosynthesis of new glycans, and the enzymatic or shear-dependent alterations in its composition. Flow-dependent reductions in the endothelial surface layer may thus affect the resistance to flow and/or the adhesion of red cells and/or leukocytes to the endothelium. Thus, future studies aimed at the molecular rheology of the endothelial surface layer may provide new insights into determinants of the resistance to flow. [source] Biological properties of a specific G,q/11 inhibitor, YM-254890, on platelet functions and thrombus formation under high-shear stressBRITISH JOURNAL OF PHARMACOLOGY, Issue 1 2006Toshio Uemura 1The effects of YM-254890, a specific G,q/11 inhibitor, on platelet functions, thrombus formation under high-shear rate condition and femoral artery thrombosis in cynomolgus monkeys were investigated. 2YM-254890 concentration dependently inhibited ADP-induced intracellular Ca2+ elevation, with an IC50 value of 0.92±0.28 ,M. 3P-selectin expression induced by ADP or thrombin receptor agonist peptide (TRAP) was strongly inhibited by YM-254890, with IC50 values of 0.51±0.02 and 0.16±0.08 ,M, respectively. 4YM-254890 had no effect on the binding of fibrinogen to purified GPIIb/IIIa, but strongly inhibited binding to TRAP-stimulated washed platelets. 5YM-254890 completely inhibited platelet shape change induced by ADP, but not that induced by collagen, TRAP, arachidonic acid, U46619 or A23187. 6YM-254890 attenuated ADP-, collagen-, TRAP-, arachidonic acid- and U46619-induced platelet aggregation with IC50 values of <1 ,M, whereas it had no effect on phorbol 12-myristate 13-acetate-, ristocetin-, thapsigargin- or A23187-induced platelet aggregation. 7High-shear stress-induced platelet aggregation and platelet-rich thrombus formation on a collagen surface under high-shear flow conditions were concentration dependently inhibited by YM-254890. 8The antithrombotic effect of YM-254890 was evaluated in a model of cyclic flow reductions in the femoral artery of cynomolgus monkeys. The intravenous bolus injection of YM-254890 dose dependently inhibited recurrent thrombosis without affecting systemic blood pressure or prolonging template bleeding time. 9YM-254890 is a useful tool for investigating G,q/11 -coupled receptor signaling and the physiological roles of G,q/11. British Journal of Pharmacology (2006) 148, 61,69. doi:10.1038/sj.bjp.0706711 [source] Heel skin hyperaemia: direct compression versus vascular occlusionCLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 6 2003Harvey N. Mayrovitz Summary Vulnerability of the heel to ulceration in bed-bound persons is related to direct pressure-induced blood flow decreases. Periodic pressure reduction is a clinical strategy to help prevent ulcers by allowing flow-repayment hyperaemia that has a magnitude and duration thought to be related to the duration of the prior interval of ischaemia. However, there are reasons to question whether effects of flow stoppages caused by direct tissue loading are similar to those because of ischaemia without superimposed direct pressure. This question was investigated by comparing posterior heel skin blood flow responses via laser-Doppler perfusion monitoring of 27 supine-lying subjects in whom blood flow was reduced by 5-min of direct heel loading on a support surface and by 5-min of ankle-cuff compression. Results showed that blood flow reductions were the same for both methods but the hyperaemia was significantly greater when flow reduction was produced by direct heel loading. This was true for ratio of peak hyperaemic flow to baseline (8·20 ± 1·32 s versus 4·68 ± 0·80 s, P,0·001), hyperaemic to baseline 3-min flow-time area ratios (4·70 ± 0·65 s versus 1·95 ± 0·29 s, P,0·001) and for total hyperaemia durations (352 ± 39 s versus 181 ± 14 s, P<0·001). These findings raise new questions regarding the precise physiological effects of heel and tissue loading in general, the factors that contribute to the hyperaemic response and their clinical impact and interpretation. Possible sources of the observed greater post-loading hyperaemia responses are discussed. [source] | |||||||||||||